All prophecies are true, or so we would be led to believe from reading or watching fiction. Apparently, this is one of the rules. Publishers expect prophecies to come true. Now, I don’t think they would have a problem if the story is about what happens if people have been expecting a prophecy to come true, but then it doesn’t. That could be an interesting story. I suspect that the real problem is when the protagonist chooses to ignore the prophecy and it doesn’t come true.
In real life, not all prophecies come true. We know that, so it shouldn’t be a big deal if a fictional prophecy doesn’t come true. But if it isn’t a big deal, then what impact does it have on the plot? An unfulfilled prophecy that the protagonist has ignored and no one is going to get upset over when it is proven incorrect is a non-event. Things are much more interesting when the prophecy comes true. Our protagonist may still ignore it, but we keep seeing these signs that the prophesied fate is coming. A true prophecy gives us foreshadowing and tension as the protagonist struggles against the inevitable. So, while we know that not all prophecies are true, false prophecies don’t add anything to the story.